In order to maximize utilization of a process module, we need to minimize overhead time spent on exchanging carriers into and out of the process module, minimize the time for keeping the process chamber door open, and improve wafer heating and cooling cycles. In substrate processing there are several possible system architectures. One of the more common is the cluster tool arrangement, in which multiple processing modules and one or more load locks are all clustered around a central transfer chamber. Other systems may employ an in-line arrangement with load locks on each end of a string of one or more processing modules, as for example in U.S. Pat. No. 6,679,671 to Blonigan et al. or in U.S. Pat. No. 6,213,704 to White et al. The environment within the respective modules and load locks are isolated from each other and from the exterior environment by gates that open only for wafer transfer when certain environmental conditions (gas composition, pressure, temperature, etc.) in adjacent modules have been made sufficiently compatible to prevent cross-contamination.
In U.S. Pat. No. 6,688,375, Turner et al. describe a load lock chamber having a two-level transport with heating elements incorporated into the transport's upper level and with cooling elements incorporated into the transport's lower level. A thermal barrier is provided between the two levels of the transport. Loading a wafer into the upper level allows that wafer to be preheated prior to moving into a process module. The lower level allows for cooling of wafers received from a process module prior to unloading.
Load locks are relatively expensive for what they accomplish. However, given the requirements of overall wafer throughput, in-line system configurations are generally acknowledged in the technology field to require two such load locks, one at each end of the processing line. Elimination of one such load lock from such substrate processing systems would reduce cost of the system, and could even reduce per unit cost of processed substrates if it could be done without substantially reducing the overall throughput. Finding a way to minimize throughput loss from any such system configuration change is desired.